Sensors which generate their (actual) output signal within the framework of a compensation method in a control process, are widely known.
For instance, EP-B 0 706 648 describes a method which is also known under the name “Halios” and can be applied to optical, capacitive and inductive sensor systems.
Halios Realization with RC Low-Pass Filters
The mode of operation of a capacitive sensor based of these measurement methods is schematically illustrated in FIG. 1. At the nodes MODP and MODN, two push-pull signals which can be modulated in their amplitude, are fed into a measurement bridge consisting of two low-pass filters. The synchronous demodulator evaluates the amplified signal with regard to the residual signal so that the controller can minimize this residual signal by amplitude modulation. In case that both bridge arms are identical, the signal will be canceled at the node CM.
If such a cancellation holds true at any point of time, the signal is called a zero signal.
In dependence on the physical measurement principle and on the specific measurement method, the zero signal which is realized will be of a better or worse quality. In the above mentioned original, i.e. optical Halios method, for instance, a very good (true) zero signal is achieved. The measurement pulses in all of their varieties (transmitting current, emitted and received light, photocurrent in the photodiode, voltage at the amplifier) largely maintain their shape, notably a rectangular shape.
In principle, the half bridge in FIG. 1 allows for the implementation of the Halios method on a capacitive basis. However, the signal existent at the amplifier output will normally not be a true zero signal anymore. Complete cancellation is obtained only in the compensated state of the measurement bridge. Should the bridge be misadjusted, i.e. should CMESS and CREF be different from each other, the controller will merely be able, by changing the amplitudes, to perform a counterbalancing control of the measurement loop for the detection times.
At the output of the amplifier, there is generated the curve development shown in FIG. 2. The quite distinct “residual signal” clearly delimits the (linear) amplification of this signal and thus also the sensitivity of the sensor.
Halios Realization with Longitudinal Capacities and Charge Amplifier
A better accuracy of the signal shape can be obtained in that the measurement and reference capacities are not arranged according to ground but, instead, are inserted “longitudinally”. If the amplifier is provided with a substantially capacitive feedback, the charge amplifier shown in FIG. 3 is obtained.
Excitation of the half bridge by square wave signals will result in square wave signals also at output of the amplifier. The controller can control the difference signal to the effect that it will be a true zero signal. The amplification of the amplifier stage can be selected to be high so that the sensor can become very sensitive. Of advantage for the practical realization is also the fact that the parasitic capacity of the CM node against ground does not play any role anymore.
Improvement of EMV
The measurement principle of capacitive sensor technology is based on the influence that the measurement object has on the measurement capacities and respectively on the electric fields in these capacities. After all, this sensor technology is in principle receptive to electromagnetic radiation from outside.
Along with the increase of the sensitivity of the actual sensor surfaces (e.g. by enlarging these surfaces), also the sensitivity to interference from outside will generally increase by the same extent. For the practical use, it is essential that this disadvantage is eliminated or significantly mitigated.